Liquid crystal polymers capable of forming an anisotropic melt phase have a number of notable characteristics including high strength, high rigidity, high resistance to heat, and good moldability. However, they exhibit different molding shrinkages and mechanical properties in the molecular chain orientation direction as compared to in the direction perpendicular. Their high prices are also disadvantageous in terms of the commercial aspect.
On the other hand, thermoplastic resins not forming an anisotropic melt phase are relatively inexpensive, but are inferior to liquid crystalline polyester in terms of resistance to heat, rigidity, and like properties. Particularly when thermoplastic resins are attempted to be used for forming a thin-wall housing, their shortage of melt resin fluidity and rigidity during manufacture of the housing limits the design to only thick walls. Therefore, the recent trend of reducing size and weight in the fields of electrical devices, electronics, and telecommunications has faced a limitation.
Accordingly, it has been attempted to use a liquid crystal polymer and a thermoplastic resin in combination so as to compensate their respective drawbacks while making best use of their advantages. However, when an injection-molded article is formed of a thermoplastic resin composition prepared by simply blending the two components, characteristic features of liquid crystal polymers such as high strength, high rigidity, high resistance to heat, and good moldability (i.e., high fluidity) are not ensured, and the mechanical strength of the article is significantly degraded. This is because a molded article of a thermoplastic resin composition resulting from a simple blending of a thermoplastic resin and a liquid crystal polymer has a structure in which almost all the liquid crystal polymer in the form of spheres except in a surface phase is merely dispersed in a thermoplastic resin matrix, and therefore, no reinforcing effect can be expected; high mechanical strength and other notable properties of liquid crystal polymers are derived from their molecular orientation when the molecules undergo shear stress and elongation stress during processing in the molten state.
When the proportion of a liquid crystal polymer is increased while that of a thermoplastic resin is decreased, the liquid crystal polymer comes to be in the form of a matrix, with the thermoplastic resin dispersed therein as islands, and this composition cannot make best use of the advantages of the thermoplastic resin, and therefore, has insignificant value in use.
Under the above circumstances, the methods as described in Japanese Patent Application Laid-Open (kokai) No. 5-70700 and No. 5-112709 were proposed. According to the methods, a mixture of liquid crystal polymer and a thermoplastic resin is extruded while the mixture is stretched at a temperature at which both are molten, to thereby prepare a material to be molded such that the liquid crystal polymer is present in advance as fibers having a large aspect ratio (length/thickness), and when a molded article is formed, the material is molded at a temperature at which the liquid crystal polymer is not molten but the thermoplastic resin is molten, to thereby obtain a molded article containing fibrous liquid crystal polymer having reinforcing effect.
In these methods, through preliminary extrusion while stretching and through elongation of the melt-extruded product by use of rollers or similar means, the liquid crystal polymer is caused to align in the composition as fibers, and subsequently, when a molded product is formed by injection molding or other process, the composition is molded at a temperature lower than the melting point of the liquid crystal polymer. Alternatively, in the direct formation of a molded product, a considerably great shear force must be applied to the resin composition when the mold is filled with the composition, to thereby orient the liquid crystal polymer. Thus, in the former case, it often occurs that fluidity is lost, molding conditions are limited, and rigidity of the resultant molded article is not satisfactory. In the latter case, the shape of the molded article is restricted, and in addition, strength may be insufficient due to lack of full orientation depending on the position of the molded article.